In automation technology, industrial control systems are used for installation, machine and process control, for example in order to control and monitor production processes or packaging processes. An industrial control system generally has a field level or process level with field devices and a control level with control devices. In industrial process automation, actuators, such as drives or valves, and measuring transducers, such as coders and sensors, are used, inter alia, as field devices.
The control level comprises, inter alia, devices for programming, controlling, monitoring and visualizing the processes to be carried out. In this case, programmable logic controllers (PLC) or control computers, for instance industrial PCs, are generally used. A control computer may be used, for example, as a process control computer on a production line.
Field bus systems which transmit communication data via a data network are usually used for communication between the control level and the field level. The communication data may be, inter alia, control commands for controlling field devices or else measurement data. The measurement data may be based, for example, on measurement variables which have been previously recorded by a field device. Field bus systems use different protocols to transmit data. One possible data transmission protocol is the EtherCAT protocol which enables real-time communication via an Ethernet network and is specified in the IEC 61158 standard. In the case of data transmission according to the EtherCAT standard, data messages are used to transmit the communication data. The data messages contain control data, which control the distribution and processing of the data messages in the field bus system, and useful data which comprise, inter alia, the communication data.
Field bus systems may be configured in different network topologies which describe the arrangement of the individual bus subscribers inside the field bus system. In the case of a ring topology, all bus subscribers are successively connected in series. Data packets containing the communication data pass through, starting from a first bus subscriber, all other bus subscribers in sequence which can replace communication data with the data packets. The data packets are then transmitted back to the first bus subscriber again. A variant of the ring topology is the line topology which represents an open ring. After the data packets have passed through all bus subscribers in sequence, the data packets are transmitted back to the first bus subscriber in the opposite direction via the bus subscribers. The first bus subscriber is generally a device on the control level, for instance a control computer.
A field bus system with real-time capability in a line topology can be implemented using the EtherCAT standard described in the IEC 61158 standard. In the case of the EtherCAT standard, the Ethernet standard or the E-bus standard, which is based on low-voltage differential signaling (LVDS), can be used, inter alia, on the physical layer. LVDS is a conductor-connected or wired transmission technology which uses differential voltage levels to transmit data and is specified in the ANSI/TIA/EIA-644-1995 standard. In the case of E-bus systems, the transmission of data is based on a current flow through a two-wire transmission line, which current flow is generated by a constant current source, binary logic values being represented by the direction of current flow. The impedance of the transmission line is generally approximately 100-120 ohms. LVDS can be used to achieve data rates of several 100 Mbit/s or some Gbit/s. Data messages which encapsulate the EtherCAT data messages are used to transmit the communication data in an E-bus system.
The different field devices used for process control, such as drives or sensors, generally do not have their own integrated interface to the field bus. They are therefore not directly controlled via the digital communication data, but rather transmit or receive digital or analog field signals. Field signals can also be called process signals. The field signals may be, for example, analog or digital voltage levels. The individual field devices accordingly usually have one or more digital or analog inputs and outputs. If the field devices are in the form of motors or drives, they can also be controlled using regulated currents which can also transmit high drive powers under certain circumstances.
Control modules are usually used to convert the communication data into the field signals. The control modules are connected both to the devices on the control level via the field bus system and to the field devices via field signal lines. The control modules may be in the form of digital or analog input modules or may be in the form of digital or analog output modules.
If they are output modules, the control modules receive control commands from the devices on the control level as communication data via the field bus system. On the basis of the control commands, the output modules generate outgoing field signals which may consist, inter alia, of analog or digital voltage signals or regulated currents. The field signals are output via output connections of the control modules. The outgoing field signals are then transmitted to the field devices via the field signal lines. The outgoing field signals may be used, inter alia, to control drives, motors or valves.
If the control modules are input modules, the control module receives incoming field signals which may consist, inter alia, of analog or digital voltage or current signals. The input module converts the field signals into communication data which may represent, for instance, measured voltage or current values. The control module can then transmit the communication data to the devices on the control level, for example to a control computer, via the field bus system.
The control modules typically provide a plurality of connections for the field signals. In this case, a control module may have only connections of one type, for instance only input connections or only output connections or only digital or only analog connections. If both analog and digital field signals are needed to operate a field device or if both inputs and outputs are needed to operate a field device, the corresponding signal lines are usually connected to a plurality of different control modules. The signal lines of different control modules which are required for a field device or for a group of field devices can be combined to form signal line bundles. The individual signal line bundles are then laid to the different field devices. The practice of combining the individual signal lines to form field-device-specific signal line bundles is also referred to as signal routing or signal distribution.
Coupling units which accommodate a plurality of control modules on a mechanical carrier are used, inter alia, to hold the control modules. The control modules are generally in the form of top-hat rail terminals and are mounted beside one another on a coupling unit in the form of a top-hat rail. Top-hat rails are described in the DIN EN 6071 standard, for example. The data connection between the control modules in the form of top-hat rail terminals is generally effected using contact devices fitted to the side of the control modules. If the control modules on the top-hat rail are pushed together, electrical contact for the purpose of communicating data is established via the contact devices of adjacent modules. In a similar manner, voltage can also be supplied to the control modules via contact devices fitted to the side of the control modules.
In the case of control modules in the form of top-hat rail terminals, the field signals for the field devices are generally output or read in at connections which are arranged on a front side of the control modules which faces away from the top-hat rail. The connections are generally in the form of spring loaded contacts. In order to combine all field signals intended for a field device or a group of field devices during signal routing, signal lines connected to the connections can be cross-wired to field-device-specific connection elements. In this case, the field-device-specific connection elements combine all signal lines intended for a field device or a group of field devices and are generally likewise in the form of spring loaded contacts.
Such top-rail terminals are previously known, for example, from DE 44 02 002 B4.